1. Field of Invention
The present invention relates to a flexible multi-layer gasketing product having high thermal stability and good sealing properties. More particularly, the present invention relates to an improved multi-layer gasketing product that does not require employment of adhesive between the layers and which comprises a non-expanded graphite layer and a fibrous layer.
2. Background of the Related Art and Information Disclosure Statement
Gasketing products are used in many applications to seal adjacent surfaces so as to prevent fluid leakage. A variety of materials are used in gasket manufacture, the particular materials used in the fabrication being chosen to meet the particular conditions under which the gasketing product is to perform. For example, vegetable fiber-based gasketing is often used to seal hot and cold liquids in applications wherein the pressure is relatively low (less than 300 psi). On the other hand, graphite-based gasketing is frequently used in pumps where the gasket may be exposed to relatively high pressure (up to 10,000 psi) and/or may be exposed to corrosive media. Neoprene-based gaskets are frequently employed in applications wherein compressibility is a factor, such as when two irregular joint flanges are to be joined together to effectuate a seal. Polymeric aromatic polyamide-based gasketing (such as aramid or Kevlar(copyright)) with a nitrile or SBR binder, offers good temperature and pressure resistance lending itself to be used for steam, gases and mild caustic sealing.
There are a number of characteristics that are typically desired in a gasketing product: (1) the material comprising the gasket should be relatively inert to prevent reaction of the gasket with its ambient environment and the surfaces it is designed to seal; (2) the material comprising the gasket should not stick to the parts against which the gasket is intended to bear; (3) the material comprising the gasket should have high wear resistance to provide for a gasket with a relatively long service life; (4) the material should be easily handable and not friable in nature; (5) the material should provide for compressibility of the gasket; and the (6) the materials comprising the gasket should be relatively inexpensive.
One particularly useful material for fabricating gaskets is graphite. Graphite is used in gasketing products because of its relative non-reactivity to other chemical compounds, its ability to resist both corrosive and non-corrosive media, its low coefficient of friction, and its excellent thermal stability.
Graphite is made up of substantially flat planes of hexagonally arranged carbon atoms, which are oriented so as to be substantially parallel and equidistant to one another. Natural graphites may be characterized as superposed laminated structures of carbon atoms joined together by weak Van der Waals forces. The laminae are linked together in groups known as crystallites.
In order to be useful in most sealing applications, graphite-based gaskets must be flexible. So-called xe2x80x9cflexible graphitexe2x80x9d is therefore conventionally used in most graphite containing gaskets. The term xe2x80x9cflexible graphitexe2x80x9d refers to the exfoliated reaction product of rapidly heated natural graphite particles which have been treated with an agent that intercalates into the crystal-structure of graphite to expand the intercalated particles at least 80 or more times in the direction perpendicular to the carbon layers in the crystal structure. A preferred intercalating agent is a solution of a mixture of sulfuric acid and an oxidizing agent. A conventional process for producing a flexible graphite sheet is described in U.S. Pat. No. 3,404,061 to Shane et al. Such process comprises treating the natural graphite particles with a suitable oxidizing agent, heating the treated particles to permit a natural expansion and then compressing and compacting the heated particles to form a flexible integrated sheet. Typical oxidizing agents include nitric acid, potassium chlorate, hydrogen peroxide, chromic acid, potassium permanganate, potassium chromate, potassium dichromate, and perchloric acid.
Flexible graphite-based gasketing products exhibit excellent temperature resistance, maintaining integrity from cryogenic temperatures to temperatures over 5400xc2x0 F. in inert or reducing atmospheres, and up to approximately 850xc2x0 F. in oxidizing atmospheres. Such products further exhibit very low creep relaxation (about 5%), excellent compressibility and conformability, and excellent chemical resistance (except for concentrated highly oxidizing mineral acids). Flexible graphite-based gaskets, however, suffer from a number of disadvantages, including, the adhesion of the graphite to mating surfaces, exhibition of a low resistance to compressive forces in particular after exposure to fluids such as oil or antifreeze, a lack of resiliency after compression, and a lack of inherent tensile strength which results in a tendency of such gaskets to break during handling in particular when bent.
In an attempt to take advantage of the desirable characteristics of graphite as a sealing agent, while avoiding its drawbacks, numerous manufacturers have proposed graphite-based gaskets fabricated from composite blends, and manufactured as laminates.
Gaskets made from composite materials that include graphite portend improved physical properties while generally offering the advantage of ease of manufacture. U.S. Pat. No. 4,282,288 discloses a graphite composite comprising a natural flake graphite bound with a binder such as coal tar or coal pitch which has been heat fixed. U.S. Pat. No. 4,443,517 discloses a fiber-reinforced graphite composition comprising a mass of intermeshed fibers bonded to one-another at points of contact by a binder and graphite distributed throughout the intermeshed fibers in an amount of from about 45% to about 95% of the total weight of the material. U.S. Pat. No. 5,286,574 discloses a composite gasket material, which is said to provide for, improved sealability and thermal stability. Such composite includes a fiber component, elastomeric binder component and a solid filler component that includes as a component a salt formed from a multivalent metal cation and a sterically hindered dicarboxylic acid. U.S. Pat. No. 4,529,653 discloses a non-asbestos flexible sheet material useful as a gasket which comprises graphite along with clay, silicate, cellulose fibers, starch and rubber. Such patent asserts improved flexibility and tensile strength. U.S. Pat. No. 5,902,762 discloses a sheet useful for forming gaskets formed from resin-impregnated flexible material which has embedded ceramic fibers extending from the surface of the sheet into the sheet. The ceramic fibers increase the permeability of the sheet to resin.
U.S. Pat. Nos. 4,977,205 and 5,437,920, both assigned to Frenzelit-Werke GmbH and Co. KG, describe composite sealing materials employing fibrillated organic fibers, flaked graphite and an elastomeric organic binder. U.S. Pat. No. 4,977,205 discloses a composite comprising between about 10% to about 70% by weight flake graphite having a particle size of 50 um to 250 um, between about 5% to about 20% by weight of an elastomer, and between about 5% to about 30% by weight process fiber, which preferably are aramids. U.S. Pat. No. 5,437,920 describes a composite comprising at least about 60% by weight finely divided graphite powder, from about 3% to about 5% elastomeric resin binder, and between about 2.5 to about 5.5% fiber of which about 0.5% to about 1.5% by weight is non-fibrillated fibers, and of which about 2 to about 4% by weight is fibrillated organic fibers.
Flexible graphite laminates have also been proposed for use as gaskets in various sealing applications. Laminates offer the possibility of maximizing the amount of graphite which is exposed to any conjoining surface. Many of these laminates contain metal or plastic sheets bonded between two sheets of flexible graphite material. The interlayer provides for an increase in strength and may improve the ease of handling of the laminate during cutting to form gaskets. Metal interlayers in particular offer the advantage of improved resistance to blowout, reduced high temperature creep of the gasket, and increased strength.
U.S. Pat. No. 5,722,670 discloses a graphite laminate gasket having two metallic layers interposed between graphite overlayers with each metallic layer having a different coefficient of expansion to maximize the gaskets sealing ability when placed between two opposing surfaces having different rates of expansion (and thus create shear forces which act differently against the surfaces of the interposed gasket). Laminates utilizing an interposed polymer resin coated cloth, such as polytetrafluoroethylene coated fiberglass cloth, are also known (See, e.g., U.S. Pat. No. 4,961,991). 
A problem associated with laminates employing graphite is that inevitably an adhesive material is incorporated. Adhesives may vaporize upon heating leaving a porous body, which permits leakage. Adhesives may also soften at elevated temperature and increase the probability of failure of the gasket. U.S. Pat. No. 5,830,809 proposes a laminate graphite gasket which avoids the use of adhesive comprising a flexible graphite layer to which a very thin thermoplastic polymer film, preferably under three mils in thickness, is fusion bonded. Fusion bonding of the thermoplastic polymer film to the graphite is performed by raising the temperature of the thin film at the interface thereof in contact with the flexible graphite while maintaining the interface of the thin thermoplastic polymer film not in contact with the flexible graphite at a temperature below said softening point. Unfortunately such process is limiting and does not provide a method for producing other types of advantageous laminates.
Many gaskets incorporate polymeric materials which do not have sufficient thermal resistance to withstand high temperatures encountered in certain applications, such as at the exhaust muffler of an air-cooled engine. Further, prior art gaskets comprising relatively high levels of graphite on the surface of the gasket suffer from the tendency of the surface graphite layer to crack and break when subjected to bending during handling. Breakage of the outer graphite layer may compromise the integrity of the gasket seal.
There is therefore a need in the art for a gasket which employs graphite on the surface of the gasket, displays improved strength and resiliency, reduces the tendency of the graphite surface layer to crack upon bending, and which does not require the employment of heat-sensitive adhesives.
The present invention provides an improved multi-layer gasket product, which comprises a non-expanded graphite layer and a fibrous layer. The improved gasket product provides significantly better flexibility/handability over conventional graphite-based products, reducing significantly the propensity of such gasketing product to crack or break upon bending.
An aspect of the present invention comprises a multi-layer structure wherein at least one outer layer of the laminate comprises at least about 30%, more preferably at least about 45%, and yet more preferably at least about 60% graphite. The multi-layer structure includes at least one intermediate layer in contact with the outer graphite layer that comprises between about 5-25% elastomeric polymer, and between about 5%-40% fiber. Unexpectedly, it has been found that a graphite layer comprising more than about 50% flake graphite can be adjoined to a fibrous material layer by use of pressure without the need for an adhesive. Unexpected and commercially practicable adhesion was in particular discovered when the fibrous material is in a highly fibrillated form such that it has a surface area greater than 5 m2/gm, and more advantageously, when the fibrillated fibrous material is an aromatic polyamide. More preferably, the intermediate layer of the laminate also comprises an elastomeric organic binder which may be selected from the group consisting of acrylonitrile-butadiene rubber, butyl rubber, chloroprene, ethylene-propylene-diene rubber, neoprene, polyisobutylene, polyisoprene, styrene-butadiene rubber, and the like. Nitrile-containing polymers/co-polymers were found in particular to advantageously enhance adhesion between the laminate layers while providing unexpected flexibility to the graphite layer protecting against cracking due to bending of the material. In particular, nitrile rubber was seen to provide advantageous flexibility along with advantageous laminar binding properties. A preferred nitrile rubber was polyacrylonitrile butadiene rubber. Preferred fibrous materials include aramid fiber (p-phenylene terephthalamide polymer), an organic fiber; carbon fiber, an organic fiber; and mineral wool, an inorganic fiber (mineral wool is made from molten slag, rock or glass, or a selected combination of the same, by blowing and drawing). Preferably the intermediate layer of the laminate containing the fibrous material further contains from about 10% to about 90% of a silicate, preferably aluminum silicate, and more preferably hydrated aluminum silicate, which surprisingly was found when added to the intermediate layer to improve the binding between the graphite layer and the intermediate fibrous/elastomeric polymer layer.
Also disclosed is a homogenous graphite flexible product comprising between 30% and 70% graphite flake, between 5% and 20% elastomeric polymer, between 5% and 25% fiber, and between 10% and 90% silicate, preferably aluminum silicate, and more preferably hydrated aluminum silicate. Unexpectedly good adhesion between the components was found when the fibrous material was fibrillated such that it has a surface area greater than 5 m2/gm. Advantageously, the highly fibrillated fibrous material is an aromatic polyamide. Such homogenous graphite flexible product was found to be compressible into a unified mass by calendering without the need to add adhesives.
It has been discovered that flake graphite which is not expanded or exfoliated (xe2x80x9cnon-exfoliatedxe2x80x9d flake graphite), may be advantageously employed in the present invention. Presently available commercial graphite-based gaskets, composites and laminates, typically employ expanded, or exfoliated, graphitexe2x80x94that is formed by intercalation of the flakes with oxidizing agents by way of heat treatment. The present invention goes against this convention. It also provides for the production of a flexible multi-layer gasket that does not require adhesives to be employed in order to bind layers.
In one embodiment of the present invention, there is provided a substantially adhesive-free flexible multi-layer graphite based gaskets comprising a first and second outer graphite layer and an intermediate layer intermediate to said first and second outer graphite layers, wherein said first and second outer graphite layers comprise at least about 30% to about 70% flake graphite, more preferably at least about 60% flake graphite, and said intermediate layer comprises between about 5% to about 20% elastomeric polymer and between 5 to 40% fiber. Preferably the fiber of the intermediate layer has a surface area greater than 5 m2/gm, and is preferably an aramid fiber. The first and second outer layers may further comprise fillers, including, but not limited to, hydrated aluminum silicate. Preferably the intermediate layer further comprises between about 10 to about 90% hydrated aluminum silicate.
Another embodiment of the present invention is a multi-layer gasketing product having an upper face layer and a lower face layer, each of the upper and lower face layers comprising at least about 30% to about 70% flake graphite by weight, more preferably at least about 50% flake graphite, and less than 25% by weight fiber, and at least one layer intermediate to the upper face layer and the lower face layer comprising between about 10% to about 40% by weight fiber and between about 5% to 20% by weight elastomeric polymer. Preferably the fiber of the intermediate layer is highly fibrillated, having a surface area greater than 5 m2/gm of fiber, and is an aramid fiber. Each of the upper and lower face layers may further comprises fillers, including, but not limited to, hydrated aluminum silicate. Preferably the intermediate layer further comprises between about 10 to about 90% hydrated aluminum silicate.
And yet another embodiment of the present invention is a non-asbestos homogenous gasketing product comprising between about 30% to about 70% by weight graphite flake, between 5%-20% of an elastomeric polymer, between about 5% to about 25% fiber of which 2%-10% is fibrillated highly fibrillated fiber having a surface area greater than 5 m2/gm and between about 10% to about 90% silicate. Preferably the silicate is hydrated aluminum silicate and the highly fibrillated fiber is an aramid fiber. As such composite does not require adhesive or resins for adhesion of the components, it is preferred that the product be substantially devoid of adhesives.